qom: Clean up object_property_get_enum()'s error value

object_property_get_enum() is the only object_property_FOO() that is
documented to return an undefined value on error.  It does no such
thing, actually: it returns 0 on some errors, and -1 on others.

Needlessly complicated.  Always return -1 on error, and adjust the
contract.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200917125540.597786-2-armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
diff --git a/qom/object.c b/qom/object.c
index 387efb2..cecad35 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -1564,21 +1564,21 @@
     EnumProperty *enumprop;
 
     if (prop == NULL) {
-        return 0;
+        return -1;
     }
 
     if (!g_str_equal(prop->type, typename)) {
         error_setg(errp, "Property %s on %s is not '%s' enum type",
                    name, object_class_get_name(
                        object_get_class(obj)), typename);
-        return 0;
+        return -1;
     }
 
     enumprop = prop->opaque;
 
     str = object_property_get_str(obj, name, errp);
     if (!str) {
-        return 0;
+        return -1;
     }
 
     ret = qapi_enum_parse(enumprop->lookup, str, -1, errp);